Rethinking the “Speed Limit” of Fueling
Why 90g/h Might Not Be Your Limit
As endurance athletes, we’ve long been told that 90 grams of carbohydrates per hour is the physiological limit for absorption. We often structure our nutrition plans around this ceiling, assuming anything more is just a recipe for GI distress.
New Science on High-Carb Fueling for Long Course
However, a new study published in the Journal of Applied Physiology challenges this doctrine, offering compelling evidence that going “super-high” carb (120g/h) might offer significant performance benefits—especially for those of us going long.
The Science: Efficiency vs. The Empty Tank
This research is exciting because it changes how we think about fatigue. The old model was simple: you get tired because you run out of fuel (glycogen depletion). We treated fueling like filling a gas tank to prevent it from running dry.
This new study proposes a regulated model rather than a failure model. It shows that high-carb intake (120g/h) doesn’t just keep the tank full; it actually makes the engine run cleaner.
Researchers found that athletes consuming 120g/h used significantly less oxygen to run at the same speed compared to those consuming 60g/h or 90g/h. This is a massive finding. It means the performance advantage isn’t just about fuel availability; it’s about efficiency.
Two Types of Efficiency
To understand why this matters, we have to look at efficiency in two ways:
- Oxidation Efficiency: What proportion of the carbohydrates you eat do you actually burn? Typically, this percentage drops as you eat more. However, even if the percentage drops, the total amount of fuel reaching your muscles is still higher at 120g/h.
- Running Economy (Oxygen Cost): This is the game-changer. Deriving energy from carbohydrates requires less oxygen than deriving it from fat. By forcing the body to rely more on carbohydrates, you lower the oxygen cost of the effort. The study found this benefit was comparable to the efficiency gains of wearing carbon-plated “super shoes.”
What This Means for Triathletes & Gravel Racers
While this study was on runners, the implications for multisport and gravel athletes are arguably even stronger.
- The “Free Speed” of Economy: In a sport where efficiency is king—whether you’re 4 hours into a gravel race or on the back half of an Ironman bike leg—spending less energy to hold the same power output is a massive advantage.
- You Have an “Iron Stomach” Advantage: The runners in the study did experience significant GI distress (nausea, fullness) at the 120g/h dose. However, cyclists typically tolerate high-carb loads better than runners because there is less gastric jostling. For a gravel racer or triathlete on the bike leg, hitting 100-120g/h is likely more achievable.
- Sparing the Tank for the Run: For triathletes, the bike leg is essentially a fueling competition. The study showed that 120g/h allowed athletes to rely almost entirely on exogenous (eaten) carbs rather than dipping into their limited internal stores. This means you arrive at T2 with fuller glycogen stores, ready to attack the run.
The Bottom Line: Train Your Gut
For performance-aspiring athletes, more carbohydrates will make the same pace feel easier because you are physically more efficient. The perception of effort—fundamental to pacing—will be lower.
However, this isn’t a license to mix a 120g bottle for your next race without prep. The study emphasized that “gut training” is essential. Start by gradually increasing your hourly intake while training—moving from 90g to 100g, then 110g—to see if you can unlock that extra gear of efficiency.